Method of preparing the surface of articles of magnesium and magnesium base alloys for the reception of nickel electroplate



United States Patent METHOD OF PREPARING THE SURFACE OF ARTICLES OF MAGNESIUM AND MAGNE- SIUM BASE ALLOYS FOR THE RECEPTION OF NICKEL ELECTROPLATE Herbert K. De Long, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application February 25, 1952, Serial No. 273,354

4 Claims. (Cl. 11762) The invention relates to the surface treatment of articles of magnesium and magnesium-base alloys. It more particularly concerns a method of preparing the surface of articles of the aforesaid metals whereby the articles can be given an electroplate of nickel from conventional acid fiuorinated nickel plating baths.

Articles of magnesium and the magnesium base alloys are readily attacked by acid solutions. As a consequence, it has not been possible to form electroplates of nickel on such articles using conventional acidulated nickel plating baths. Accordingly, it is the principal object of the invention to provide a method of treating articles of magnesium and the magnesium base alloys containing at least 85 percent of magnesium so as to render the surface electroplateable from an acidulated nickel plating bath.

In brief, the method of the invention comprises treating the cleaned article with an aqueous solution containing both copper sulfate and chromic acid followed by rinsing in water and a treatment in an aqueous acid fluoride solution. Articles of magnesium and magnesium-base alloys so treated, may be electroplated in conventional manner in conventional acidulated nickel electroplating baths. The invention then consists of the method herein fully described and particularly pointed out in the claims.

Prior to carrying out the method, the surface of the article to be treated is subjected to a suitable conventional cleaning operation to remove surface contaminants such as grease, oil, oxide etc. Grease and oil are best re moved by a cathodic electrolysis for 3 to 5 minutes at 180 to 212 F. in an aqueous alkaline solution of the following composition, although other compositions may be used: trisodium phosphate (NasPOs' H2O) 30 grams per liter, sodium carbonate (Na2CO3'10H2O) 30 grams per liter, a wetting agent e. g. an alkyl aryl sulfonate 0.5 gram per liter. After removal of the grease and oil, it is desirable to remove any oxide which may be left on the article, as by a pickling for 30 seconds at room temperature in a 5 percent aqueous solution of nitric acid and then rinsing in water.

In accordance with the invention, the cleaned article is subjected to the chemical action of an aqueous solution containing from 30 to 100 grams per liter of copper sulfate (011504) and from 6 to 30 grams per liter of chromium trioxide (CI'Os). This operation is best performed, by keeping the article submerged in the solution for 5 to 60 seconds, the solution having a temperature of 20 to 30 C., and a pH of at least 2 but not more than 6 and preferably not over 4. In this operation, a thin layer of copper forms on the surface of the article. As

rinsed article is then immersed or otherwise subjected to the action of an aqueous solution containing HF. The concentration of the HF is not sharply critical. The HF may be derived from commercial aqueous hydrofluoric acid which nominally contains 48 percent of HF by weight. This hydrofluoric acid solution (or other suitable acid fluoride solution) may be suitably diluted to concentrations providing from 17 to 280 grams of HF per liter. Acid fluorides may be used as a source of HF, such as the alkali metal and ammonium acid fluorides, e. g., NaF-HF, NH4F-HF. The acid fluorides may be used in amount ranging from 50 to 200 grams per liter. A preferred concentration for the HF is about 15 percent HF by weight or about 160 grams of HF per liter of the solution as derived from hydrofluoric acid.

After immersing the copper coated article in the HF containing solution, the so treated article is removed and rinsed in water. The wet article is then ready for electroplating in conventional manner.

The following example is illustrative of the invention:

A piece of sheet 3 x 4 x 0.062 inch of a magnesium base alloy having the nominal composition of 3 percent A1, 0.3 percent Mn, 1 percent Zn, the balance magnesium, was cleaned in a conventional alkaline cleaning solution, pickled for 30 seconds in 5 percent aqueous nitric acid solution at room temperature, and then rinsed in water. The surface of the piece was thereby rendered clean and bright in appearance. The cleaned piece was then subjected to treatment according to the invention. This consisted in transferring the piece while still wet from the foregoing rinsing to an aqueous solution containing 50 grams of CuSOr per liter and 9 grams of CrOs per liter of the solution. The piece was left in the solution for 30 seconds then withdrawn and rinsed in water. Following the rinsing, the piece appeared to have a dark grey to black uniform amorphous coating. The so appearing piece, while still wet from rinsing, was immersed in a 15 percent aqueous solution of hydrofluoric acid for 60 seconds at room temperature and then withdrawn and rinsed in water. The rinsed coating had the characteristic color of metallic copper. Further examination of the surface of the so treated piece by means of the electron microscope, showed that the surface was coated with metallic copper. The rinsed piece was then electroplated with nickel by electrolyzing a conventional nickel plating bath with the piece as cathode using a current density of 20 amperes per square foot for 20 minutes at 40 C. The plating bath contained 7.9 oz. per gallon of nickel sulfate (NiSO4-7HzO), 6.8 oz. per gallon of ammonium acid fluoride (NH4F-HF), 4.6 oz. per gallon of boric acid (HaBOs), and 0.13 oz. per gallon of a wetting agent of sodium lauryl sulfate, the pH of the plating bath being about 5.8 to 6.0. After the nickel plating operation, the plated piece was rinsed and dried. Examination revealed that a uniform and adherent plating of nickel was obtained.

I claim:

1. In a method of treating the surface of an article of magnesium and the magnesium base alloys containing at least percent of magnesium so as to render the surface receptive to electrodeposition of a uniform adherent article to the action of an aqueous" solution containing from 30 to 100 grams of copper sulfate (CuSO4) and from 6 to 30 grams of chromium trioxide (CrOa) per liter the pH of the solution being from 2 to 6, so as to form a copper containing coating on the article, rinsing the so treated article in Water, and then subjecting the article to the action of an aqueous acid fluoride solution containing from about 17 to 280 grams of HF per liter of solution.

2. In a method according to claim 1 in which the aqueous acid fluoride solution comprises hydrogen fluoride.

3. In a method" according to claim 1 in which the aqueous acid fluoride solution comprises" sodium acid fluoride.

4. In a method according to claim 1 in which the aqueous" acid' fluoride" solution comprises ammonium acid' fluoride.

References Cited in the file of this patent UNITED STATES PATENTS 2,313,756 Loose Mar. 16, 1943 2,526,544 De Long Oct. 17, 1950 2,560,979 Pessel July 17, 1951 FOREIGN PATENTS 538,616 Great Britain Aug. 11, 1941 584,233 Great Britain Jan. 9, 1947 

1. IN A METHOD OF TREATING THE SURFACE OF AN ARTICLE OF MAGNESIUM AND THE MAGNESIUM BASE ALLOYS CONTAINING AT LEAST 85 PERCENT OF MAGNESIUM SO AS TO RENDER THE SURFACE RECEPITIVE TO ELECTRODEPOSITION OF A UNIFORM ADHERENT PLATING OF NICKEL, THE STEPS WHICH CONSIST IN SUBJECTING THE ARTICLE TO THE ACTION OF AN AQUEOUS SOLUTION CONTAINING FROM 30 TO 100 GRAMS OF COPPER SULFATE (CUSO4) AND FROM 6 TO 30 GRAMS OF CHROMIUM TRIOXIDE (CRO3) PER LITER THE PH OF THE SOLUTION BEING FROM 2 TO 6, SO AS TO FROM A COPPER CONTAINING COATING ON THE ARTICLE, RINSING THE SO TREATED ARTICLE IN WATER, AND THEN SUBJECTING THE ARTICLE TO THE ACTION OF AN AQUEOUS ACID FLUORIDE SOLUTION CONTAINING FROM ABOUT 17 TO 280 GRAMS OF HF PER LITER OF SOLUTION. 